(a) Field of the Invention
The present invention relates to an organic thin film transistor (TFT) having an organic semiconductor layer (organic film), more specifically to the organic TFT operating at a higher speed and a higher frequency.
(b) Description of the Related Art
A TFT is widely used as a switching device in display device such as an LCD. Conventional TFTs are made of amorphous or polycrystalline silicon. A CVD apparatus for fabricating the TFTs is expensive, and the fabrication of the large-sized display having the TFTs accompanies significant increase of the fabrication cost. Because of the film formation of the amorphous or polycrystalline silicon conducted at a higher temperature, some kinds of substrates, such as a resin substrate, cannot be used in the TFT or excludes a light-weight resin substrate
A TFT using organic compounds in place of the amorphous or polycrystalline silicon has been proposed for overcoming the above problems. The vacuum deposition technique and the coating technique are known for forming the film by using the organic compounds. These techniques can realize the larger devices while suppressing the cost increase, and in addition can decrease the temperature required for the film formation. Accordingly, the restriction of the substrate materials is advantageously alleviated in the TFT using the organic compounds, and the practical application thereof is expected.
In recent years, the TFTs having the organic compounds have been frequently reported. Examples thereof include Journal of Applied Physics, 54, p3255, 1983 (F. Ebisawa, et.al), Applied Physics Letter, 53, p195, 1988 (A. Assadi, et.al), Chemical Physics Letter, 167, p503, 1990 (G. Guillaud, et.al), Applied Physics Letter, 57, p2013, 1990 (X.Peng, et.al), Synthetic Metals, 41 to 43, p1127, 1991 (G. Horowitz, et.al), Synthetic Metals, 41 to 43, 1991 (S. Miyauti, et.al), Applied Physics Letter, 63, p1372, 1993 (H. Futigami, et.al), Applied Physics Letter, 62, p1794, 1993 (H. Koezuka, et.al), Science, 265, p1684, 1994 (F. Garnier, et.al), Synthetic Metals, 68, p65, 1994 (A. R. Brown, et.al), Science, 268, p270, 1995 (A. Dodabalapur, et.al), Synthetic Metals, 86, p2259, 1997 (T. Sumitomo, et.al), Thin Solid Films, 331, p51, 1998 (K. Kudo, et.al). Synthetic Metals, 102, p900, 1999 (K. Kudo, et.al) and Synthetic Metals, 111 to 112, p11, 2000 (K. Kudo, et.al.
Dimers to polymers such as conjugated polymers and thiophenes (JP-A-8(1996)-228034, 8(1996)-228035, 9(1997)-232589, 10(1998)-125924 and 10(1998)-190001), metal phthalocyanine compounds (JP-A-2000-174277), and condensed aromatic hydrocarbons such as pentacene (JP-A-5(1993)-55568 and 2001-94107) are used as the organic compound of the TFT, in the form of a mixture with other compounds or a single substance.
As shown in FIG. 1, an organic TFT 20 includes a gate electrode (layer) 24 and a dielectric layer 26 in this turn on a substrate 21. A source electrode 22 and a drain electrode 23 are separately mounted on the dielectric layer 26. An organic film 25 is formed on the surfaces of the electrodes 22,23 and on the exposed surface of the dielectric layer 26 between the electrodes 22,23. In the organic TFT 20 having the above configuration, the organic film 25 forms a channel region, and the on/off operation is conducted by controlling current flowing between the source electrode 22 and the drain electrode 23 by means of a voltage applied to the gate electrode 24.
In the conventional organic TFT 20, the carrier (free electrons or holes) moves in a direction of the surface of the organic film 25 shown by an arrow xe2x80x9cAxe2x80x9d in FIG. 1 and the channel length is too long for carrier mobility in the channel region made of organic compounds. Accordingly, the high-speed and high-frequency operation has not be realized. Further, the conventional organic TFT in which the carrier moves in a direction of the thickness of the organic thin film layer has increased response speed because of the shortened channel length. However, also in this case, the sufficient driving speed cannot be realized because of the insufficient mobility of the organic compound
It is therefore an object of the present invention to provide a high-speed and high-frequency organic TFT having shortened the channel length and including the compound with the higher mobility.
The inventors found that, after extensive investigations, when the carrier which moves from one electrode to the other one moves in the direction of the thickness of the organic thin film and the organic thin film of the TFT contains a compound having a specified structure in the form of a mixture or a single substance, the response speed of the organic TFT is significantly improved.
Thus, the present invention provides an organic TFT including a substrate, an organic film supported by the substrate, first and second electrodes each disposed in contact with opposite surfaces of the organic film each other, and a third electrode disposed at a specified distance from each of the first and second electrodes, the third electrode being applied with a voltage to control carriers moving from one of the first electrode and the second electrode to the other through the organic film; and the organic film including, in the form of a mixture or a single substance, a compound represented by at least one of general formulae [1] to [6], wherein each of R1 to R80 independently represents hydrogen atom, halogen atom, hydroxyl group, substituted or non-substituted amino group, nitro group, cyano group, substituted or non-substituted alkyl group, substituted or non-substituted alkenyl group, substituted or non-substituted cycloalkyl group, substituted or non-substituted alkoxy group, substituted or non-substituted aromatic hydrocarbon group, substituted or non-substituted aromatic heterocyclic group, substituted or non-substituted aralkyl group, substituted or non-substituted aryloxy group or substituted or non-substituted alkoxycarbonyl group or carboxyl group; any two adjacent groups of R1 to R80 may form a ring; xe2x80x9cL1xe2x80x9d represents substituted or non-substituted alkyl group, substituted or non-substituted alkenyl group, substituted or non-substituted cycloalkyl group, substituted or non-substituted aromatic hydrocarbon group, substituted or non-substituted aromatic heterocyclic group or substituted or non-substituted aralkyl group; xe2x80x9cL2xe2x80x9d represents substituted or non-substituted alkylene group, substituted or non-substituted alkenylene group, substituted or non-substituted cycloalkylene group, substituted or non-substituted arylene group or substituted or non-substituted aralkylene group; xe2x80x9cAr1xe2x80x9d to xe2x80x9cAr3xe2x80x9d represent substituted or non-substituted aromatic hydrocarbon group having carbons from 6 to 20; xe2x80x9cnxe2x80x9d represents an integer from 1 to 3; xe2x80x9cmxe2x80x9d represents an integer from 0 to 2; xe2x80x9clxe2x80x9d is 0 or 1; xe2x80x9csxe2x80x9d is 1 or 2; xe2x80x9ctxe2x80x9d represents an integer from 0 to 3; and xe2x80x9cMxe2x80x9d represents a metal ion having a valency of xe2x80x9cn+mxe2x80x9d or xe2x80x9cs+1xe2x80x9d.
[General Formula 1]
[General Formula 2]
[General Formula 3]
[General Formula 4]
[General Formula 5]
[General Formula 6]
In the present invention, the compound represented by the general formulae [1] to [6] can be replaced with a compound selected from substituted or non-substituted condensed aromatic hydrocarbons having carbons from 14 to 34.
In accordance with the organic TFT of the present invention, the carrier moves from one of the first and second electrodes to the other in the direction of the film thickness of the organic film. The device structure realizes the enough short channel length. The organic film containing the compound represented by at least one of general formulae [1] to [6] or substituted or non-substituted condensed aromatic hydrocarbons having carbons from 14 to 34, in the form of a mixture or a single substance provides the higher carrier mobility thereby the organic TFT with the sufficiently higher speed response is realized.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.